1. Field of the Invention
The present invention relates to nickel (Ni)-based heat-resistant alloys, and particularly to Ni-based forged alloys suitable for large members. The invention also relates to gas turbine members formed using the above Ni-based forged alloy of the invention and a gas turbine constructed using such gas turbine members of the invention.
2. Description of Related Art
An effective way to increase the efficiency of gas turbines is to increase their combustion temperature. So, many of the small parts and large members of gas turbines are made of an Ni-based heat-resistant alloy having superior high-temperature mechanical strength. Such an Ni-based heat-resistant alloy contains a high proportion of solid-solution strengthening elements (such as tungsten (W), molybdenum (Mo) and cobalt (Co)) and precipitation strengthening elements (such as aluminum (Al), titanium (Ti), niobium (Ni) and tantalum (Ta)).
A foremost precipitation strengthening phase is γ′ (gamma prime) phase (Ni3Al phase), which has a peculiar property in that its mechanical strength increases with increasing temperature. Thus, γ′ phase is extremely effective in improving the high-temperature mechanical characteristics of Ni-based alloys. The addition of elements such as Ti, Nb and Ta has been found to stabilize γ′ phase and allow γ′ phase to remain up to higher temperature. Based on this knowledge, the development of high performance Ni-based alloys has been conventionally focused on the stabilization of γ′ phase.
On the other hand, these elements such as Ti, Nb and Ta are prone to segregate during the solidification of large ingots, resulting in an inability to form large members. Thus, conventionally, the application of high-strength Ni-based alloys to turbines has been confined to relatively small turbine members such as rotor blades and stator vanes.
It is known that alloy 718 is a high-strength Ni-based alloy that can be used to form relatively large turbine members. For example, JP-A Hei 4 (1992)-280938 discloses a method for casting an Ni-based superalloy (such as alloy 718) having a composition within a specified range, in which the relationship of “G/R≧0.5 (° C.·h/cm2)” is satisfied, where R (cm/h) is a solidification rate of a melt of the Ni-based superalloy during casting and G (° C./cm) is a temperature gradient across a solidification interface. According to JP-A Hei 4 (1992)-280938, such control of the solidification rate and the temperature gradient can reduce solidification segregation, resulting in an improvement in ductility of the Ni-based superalloy.
JP-A 2008-179845 discloses a structural member made of a superalloy comprising a superalloy matrix and several types of hard nanoparticles dispersed along grain boundaries of the superalloy matrix, wherein the content of the hard nanoparticles in the superalloy structural member is from about 1 to about 30 vol. %. The superalloy structural member is formed by thermomechanically processing the superalloy matrix and the hard nanoparticles. According to JP-A 2008-179845, the superalloy structural member has an increased high-temperature stability, and thus has an increased strength and an increased fatigue strength.
Fu et al. report an improved 718 alloy obtained by compositional adjustment, wherein the improved 718 alloy contains micro-precipitation phases (γ′ phase and γ″ phase) and a new type stable spherical precipitation phase different from δ (delta) phase of conventional 718 alloys, the new type precipitation phase precipitating along grain boundaries of the improved 718 alloy (see Shuhong Fu, Jianxin Dong, Maicang Zhang, Ning Wang, and Xishan Xie: “Research on Inconel 718 Type Alloys with Improvement of Temperature Capability”, 7th International Symposium on Superalloy 718 and Derivatives, TMS, 2010). According to Fu et al., the improved 718 alloy has a longer stress rupture life and higher fatigue resistance than conventional 718 alloys.
As described before, a problem is that larger members are more difficult to manufacture by casting and forming. For example, it is probably difficult to maintain the solidification conditions described in JP-A Hei 4 (1992)-280938 uniformly throughout the entire volume of a large member of heavier than 5 tons.
Another problem is as follows: A product made of Ni-based heat-resistant alloys are forged or rolled at conventional high temperatures. In addition, an extremely large forming load is required to forge or roll a high-strength material into a large product. Therefore, large members of a high-strength material are forged or rolled at higher-than-conventional temperatures in order to reduce the deformation resistance of the high-strength material. However, frequent or incessant heating is required to maintain such a higher forming temperature, thus consuming longer processing time and larger heating energy. Also, a high-strength material is more likely to suffer from cracking during such higher temperature processing due to partial melting thereof. In terms of formability and processability, the technologies of JP-A 2008-179845 and Fu et al. may have the disadvantage of poor hot forgeability.
As described above, there is a tradeoff between the mechanical strength properties and the manufacturability (such as large ingot formability and hot formability) of Ni-based heat resistant alloys. As described, in order to increase the efficiency of gas turbines, large-size turbine members having a high-temperature mechanical strength are required. Thus, a strong demand exists for high-temperature materials having improved large ingot formability and improved hot formability.